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I don't know the answer to this question, but I'll describe an experiment first conducted in 1805 by physicist Thomas Young and later refined by physicist Pier Merli in 1974 that muddies the issue somewhat.

If you set up a source of light, like sunlight or a light bulb, such that it passes through two very small slits in a solid barrier and then shines onto a wall, the projection of the light on the wall will be in the form of alternating bands of dark and light.

This happens because the light coming out of the two slits will, on some spots on the wall, combine constructively where two wave crests meet, creating a band of light. On other spots the waves will combine destructively, where a wave crest and a wave trough meet, and cancel each other out, creating a dark band.

If you cover one of the slits, then only a broad, diffuse area is illuminated and no banding is observed. Uncover the slit and the alternating pattern of dark and light return.

Here's the weird part:

This experiment can be conducted using a device that fires off single individual photons at the slits in the barrier instead of a continuous torrent of them as in light from the sun or a light bulb.

If you fire single photons one at a time at the slits, the single photons will land on the wall only at places where the original light banding was observed and never where the dark banding was observed in the experiment above.

Which specific band they land on is random and the probabilities can be calculated using the equations of Quantum Mechanics, but this isn't important. What's important to note is that the photons somehow 'know' that they shouldn't land in some areas (the previously dark bands) and that they should land in some other areas (the previously illuminated bands) even though there are only single photons involved that can't interfere with each other in the conventional sense.

How do these photons 'know' where to go if they're not conscious or aware?

OK, now comes the really weird part:

If you cover one of the slits, the single photons begin to land randomly all over the wall in the same way that using a continuous light causes the wall to be diffusely illuminated with no banding when one of the slits is covered.

How do these photons 'know' that one of the slits is covered given that they only pass through the open slit and do not interact in any discernable way with the covered slit? Uncover both slits, and the single photons begin landing in the prescribed banding pattern again.

Given the results of this experiment, it's hard to escape the conclusion that photons are in some way 'aware' of their surroundings and know which spot on the wall to land on depending on whether one slit is covered or not.

This experiment can be done using single electrons and even single atoms and the results are always the same.

--------------------Republican Values:

1) You can't get married to your spouse who is the same sex as you.
2) You can't have an abortion no matter how much you don't want a child.
3) You can't have a certain plant in your possession or you'll get locked up with a rapist and a murderer.

Quantum mechanics is indeed crazy shit. One way of explaining the photon awareness thing is to imagine that the photon tries all possible paths of going through the slit, if one path leads to the blocked slit, it takes the path through the open slit. Another way of explaining it is to think of the photon as a 'probability wave' which propagates through the slits just like a normal wave would (it was this which made people initially think that light was a wave rather than a particle), the probability wave produces the same pattern on the wall beyond the slit as it would with millions of photons, but instead of it forming various stripes of varying light intensity, it forms stripes of varying probability, the higher the 'intensity' of the stripe means the higher the probability that the photon ends up there.

The two ways of explaining it are both equally valid, and equally crazy. I am reading a book about superstring theory. In a nutshell, superstring theory treats particles such as quarks and photons are loops of string vibrating in an 11 dimensional space-time, where the extra 7 dimensions are tightly wrapped around so that we do not notice its effects, but their influence can be felt on incredibly small scales. It is on these small scales (around 10^-30 metres) that the craziness of quantum mechanics comes into play, by introducing these extra tiny dimensions it can explain strange quantum effects such as tunneling (particles being here one moment, appearing there the other moment, in real life, like walking through a brick wall and having the brick wall not interact with you). I haven't got to the chapter which explains precisely how superstring theory deals with these quantum phenomena, but the above is a general gist of how it explains quantum mechanics in a way which is coherent with logic.

--------------------'Everything that can be counted does not necessarily count; everything that counts cannot necessarily be counted' - Albert Einstein

This experiment is not new for me, but I still every time I think or read about it.

I first got familiar with Newton and Einsteins theory. I really liked those theories because they are deterministic; everything has a cause and according to Einstein or Newtons theory we could, in theory, predict the future. I love the beauty of that.

Then I read about QM and I hated that the world is so random. I totally understand why Einstein didn't like the "god plays dice" part of QM.

I can see the beauty of QM and I love to read about those weird things, but I still prefer a deterministic theory

One way of explaining the photon awareness thing is to imagine that the photon tries all possible paths of going through the slit, if one path leads to the blocked slit, it takes the path through the open slit.

But this doesn't explain how two single-shot photons, both passing through the slit on the right, for example, each somehow know not to land where a dark band normally occurs. The banding is not a function of which slit the photon happened to pass through. A photon passing through the left slit and a following one passing through the right slit can, and sometimes do, both land on the same spot on the wall.

Another way of explaining it is to think of the photon as a 'probability wave' which propagates through the slits just like a normal wave would

This explains the behavior, but not the mechanism. How do these probabilities 'know' if they should spread out homogeneously when one slit is covered and in bands when both slits are open?

This experiment demonstrates one of the odd paradoxes of Quantum Mechanics and it has never been explained. Some physicist think the inescapable conclusion is that the photon is 'aware' of its surroundings in the conventional sense and that larger conglomerates of matter, like us (or a mouse trap), are just an extension of this idea by way of a cascade of collapsing wave functions leading to our own awareness of our surroundings.

Weird science indeed...

--------------------Republican Values:

1) You can't get married to your spouse who is the same sex as you.
2) You can't have an abortion no matter how much you don't want a child.
3) You can't have a certain plant in your possession or you'll get locked up with a rapist and a murderer.

I expect the problem arises due to our macroscopic concept of a "particle". In reality, "particles" do not exist as separate entities with well-defined locations. QM suggests that the particles are, in fact, merely probability waves. We "see" individual particles only because we are looking at individual portions of the probability wave.

Personally, I have always been drawn towards Einsteins view of the universe: that all forces/matter are waves.

--------------------

Once, men turned their thinking over to machines in the hope that this would set them free.
But that only permitted other men with machines to enslave them.

What are you reading? I love books on this topic, but they dont come out very often. The last one I read was "The Elegant Universe"...

--------------------After one comes, through contact with it's administrators, no longer to cherish greatly the law as a remedy in abuses, then the bottle becomes a sovereign means of direct action. If you cannot throw it at least you can always drink out of it. - Ernest Hemingway

If it is life that you feel you are missing I can tell you where to find it. In the law courts, in business, in government. There is nothing occurring in the streets. Nothing but a dumbshow composed of the helpless and the impotent. -Cormac MacCarthy

He who learns must suffer. And even in our sleep pain that cannot forget falls drop by drop upon the heart, and in our own despair, against our will, comes wisdom to us by the awful grace of God. - Aeschylus

But this doesn't explain how two single-shot photons, both passing through the slit on the right, for example, each somehow know not to land where a dark band normally occurs.

There is no way you can say it only goes through the right slit. So you covered up the left slit for this experiment? Well then you'll observe the single slit interference pattern.

If both slits are uncovered, then the corresponding waves from a single photon travel through BOTH slits. The resulting pattern you see with dark and light bands is due to the wave interference from the photon coming from the left slit and the photon from the right slit. The dark and light bands can be predicted from simple equations involving the distance between the slits and the wavelength of the light. There is no awareness in the photons; it's simply wave interference.

This explains the behavior, but not the mechanism. How do these probabilities 'know' if they should spread out homogeneously when one slit is covered and in bands when both slits are open?

This explains both the behavior and the mechanism. The waves travelling through the left slit and the waves travelling through the right slit go through constructive or destructive interference after passing through the slits. This causes your light and dark bands. I don't see any paradox in this...

Edit: Here's a diagram for you, showing the wave front from the source, how one photon travels through both slits, and the results wave interference resulting in the pattern. Note that a photon travels in every possible path to its target, therefore a photon can be pictured as a wavefront like in the picture.

If both slits are uncovered, then the corresponding waves from a single photon travel through BOTH slits.

But these waves are mathematical constructs used to explain observations. These waves are undetectable, they're just math. We're not talking about electromagnetic waves here.

The resulting pattern you see with dark and light bands is due to the wave interference from the photon coming from the left slit and the photon from the right slit.

This is what I'm getting at. A single photon somehow interferes with itself.

The waves travelling through the left slit and the waves travelling through the right slit go through constructive or destructive interference after passing through the slits. This causes your light and dark bands. I don't see any paradox in this...

The paradox occurs in that photons, electrons and even complete atoms demonstrate this behavior. So are atoms waves or things? This is the paradox. They appear to be both.

Probability waves. What exactly is that? There is no instrument that can detect these waves. They are just one convenient mathematical representation of what we think is happening.

I still see a paradox.

--------------------Republican Values:

1) You can't get married to your spouse who is the same sex as you.
2) You can't have an abortion no matter how much you don't want a child.
3) You can't have a certain plant in your possession or you'll get locked up with a rapist and a murderer.

Well I'm studying for an exam tomorrow, and I'll take the quote right out of the chapter I'm studying.

"Wave-Particle Duality"The fact that particles can behave like waves and that electromagnetic waves can behave like particles seems like a paradox. What really is a photon or an electron? Are they waves or particles? The answer is that entities in nature are more complex than we are used to thinking, and they have, simultaneously, both particle and wave properties. Which properties dominate, depends on the object's energy and mass."

wavelength = h / p = hc / sqrt(T^2 + 2T*mc^2)

"When the wavelength of an object is much less than atomic dimensions (~10^-10 m), it behaves more like a classical particle than a wave. However, for objects with longer wavelengths, wave properties tend to be more apparent than particle properties."

Photons are used to represent light waves, as wave properties dominate at that wavelength. It is just easier to think of lightwaves as particles for conceptualizing. Once one understands wave-particle duality, quantum mechanics really isn't that far out there.

Edit: You can associate ANY mass with a corresponding wavelength based on its mass and kinetic energy. It's just that these wavelengths are so small that they are unnoticeable for large objects like a dresser, book or a tree. So since the wavelengths are so small, they overwhelmingly take on particle properties. For classical objects,

Quote:Alan Stone said:They appear to be things when really they are waves. No paradox.

This 'wave' is a 'probability wave' which can collapse and is only a way to visualize how things work on the quantum level. There is no equation for the collapse of this wave, it is a weird transformation from a 'wave' to a 'particle'.

Are they waves or particles? The answer is that entities in nature are more complex than we are used to thinking, and they have, simultaneously, both particle and wave properties.

Hence the paradox.

Waves and particles are mutually exclusive concepts. Common sense says that something can't be both, and yet at the scale of an atom, common sense breaks down.

Common sense tells me that an object can't have awareness, but at the scale of an atom...

--------------------Republican Values:

1) You can't get married to your spouse who is the same sex as you.
2) You can't have an abortion no matter how much you don't want a child.
3) You can't have a certain plant in your possession or you'll get locked up with a rapist and a murderer.

Quote:Photons are used to represent light waves, as wave properties dominate at that wavelength. It is just easier to think of lightwaves as particles for conceptualizing. Once one understands wave-particle duality, quantum mechanics really isn't that far out there.

Photon are the messenger particle of the electromagnetic force. They will all show an interference pattern, no matter what wavelength.

I really don't understand what wave properties you mean and why they dominate at some wavelenghts?

Quote:Annom said:This experiment is not new for me, but I still every time I think or read about it.

I first got familiar with Newton and Einsteins theory. I really liked those theories because they are deterministic; everything has a cause and according to Einstein or Newtons theory we could, in theory, predict the future. I love the beauty of that.

Then I read about QM and I hated that the world is so random. I totally understand why Einstein didn't like the "god plays dice" part of QM.

I can see the beauty of QM and I love to read about those weird things, but I still prefer a deterministic theory

Personally, I love the indeterminism of quantum mechanics. Makes life a bit more exciting, IMO.

"QM's laws make no claim to describe physical reality itself, but only probabilities of the occurrence of a physical reality that we have in view. I cannot but confess that I attach only a transitory importance to this interpretation. I still believe in the possibility of a model of reality - that is to say, of a theory which represents things themselves and not merely the probability of their occurrence." - Albert Einstein, 1931

I study nuclear engineering, so there's a big emphasis on modern physics.

Wave properites are things like frequency, period, wave fronts and wave interference. Things that can be explained by wave equations involving sine and cosine.

Think of a spectrum, ranging from massive and energetic on the left to massless and very little energy on the right. It makes sense that material things should be on the left side; things like a tree, a computer, a space shuttle. On the far right will be things like lightwaves (no mass, some energy), radiowaves (no mass, some energy).

The de Broglie wavelength for a particle is given by: lambda = h / mv = hc / E

So you can have a scale on the spectrum, showing the corresponding de Broglie wavelengths. On the far left will be the extremely small wavelengths, because these things have large masses and a huge amount of energy. Then on the right side, you can represent these radio waves as a "particle." This particle's mass and energy are much smaller than the mass and energy of a space shuttle, for instance, so you will have a large de Broglie wavelength.

But this is a spectrum we're dealing with, it's not two poles; you have de Broglie wavelengths everywhere between the two extremes. The far left side of the spectrum will behave 100% like particles, or regular matter in our everyday world. The far right side will behave 100% like waves. 100% will never actually occur, I'm just using it to show the extremes. Everything else in between will have some wave properties and some particle properties; which dominates will depend on the mass and energy of the particle.

Any particle can be represented as a wave, and any wave can be represented as a particle. Which one you use just depends on what you're working on. It's easier for humans to think in terms of particles than waves, and I think that's where a lot of confusion comes up.